Coking systems are commonly used in petroleum refineries for converting vacuum tower bottoms and/or other heavy (i.e., high boiling point) residual petroleum materials to petroleum coke and other products. The greater part of each barrel of resid material processed in the coker will typically be recovered as fuel gas, coker gasoline/naphtha, light cycle oil (also commonly referred to by various other names such as light coker gas oil), and heavy cycle oil (also commonly referred to by various other names such as heavy coker gas oil).
A typical delayed coking system comprises: a combination tower or other fractionator; a fired heater; and at least one vertical coking drum. Most coking systems include at least a pair of vertical coking drums. The heavy coker feed is typically delivered to the bottom of the fractionator wherein it is combined with a heavy, liquid, residual bottom product (commonly referred to as a "recycle") produced in the fractionator. The resulting mixture is drawn from the bottom of the fractionator and then pumped through the heater and into at least one coking drum. Typically, multiple coking drums are operated in alternating cycles such that, while one drum (referred to herein as the "live" drum) is operating in a fill cycle, another drum is operating in a second cycle typically comprising a steaming stage, a cooling/quenching stage, a hydraulic decoking stage, a pressure testing stage, and a warmup stage.
In the fill cycle, the hot feed material from the coker heater typically flows into the bottom of the live coking drum. Some of the heavy feed material vaporizes in the heater such that the material entering the bottom of the coking drum is a vapor/liquid mixture. The vapor portion of the mixture undergoes mild cracking in the coking heater and experiences further cracking as it passes upwardly through the coking drum. The hot liquid material undergoes intensive thermal cracking and polymerization in the coking drum such that the liquid material is converted to cracked vapor and petroleum coke. The resulting combined overhead vapor product produced in the coking drum is typically delivered to a lower portion of the fractionator wherein it is separated into gas, naphtha, light cycle oil, and heavy cycle oil, which are withdrawn from the fractionator as products, and a heavy recycle/residual material which flows to the bottom of the fractionator. The light and heavy cycle oil products are typically taken from the fractionator as side-draw products which are further processed (e.g., in a fluid catalytic cracker) to produce gasoline and other desirable end products. The heavy recycle material combines with the heavy feed material in the bottom of the fractionator and, as mentioned above, is pumped with the heavy feed material through the coker heater.
By way of example, but not by way of limitation, typical coker operating conditions and product specifications include: a heater outlet temperature in the range of from about 905 to about 935.degree. F.; coke drum pressures in the range of from about 20 to about 40 psig; live drum overhead temperatures in the range of from about 800.degree. to about 820.degree. F.; a fractionator overhead pressure in the range of from about 10 to about 30 psig; a fractionator bottom temperature in the range of from about 750.degree. to about 780.degree. F.; a light cycle oil draw temperature in the range of from about 450.degree. to about 550.degree. F.; a light cycle oil initial boiling point (ASTM D-1186) in the range of from about 300.degree. to about 325.degree. F.; a light cycle oil end point (D-1186) in the range of from about 600.degree. to about 650.degree. F.; a heavy cycle oil draw temperature in the range of from about 600.degree. to about 690.degree. F.; a heavy cycle oil initial boiling point (D-1186) in the range of from about 470.degree. to about 500.degree. F.; and a heavy cycle oil end point (D-1186) in the range of from about 960.degree. to about 990.degree. F.
One of the most serious and commonly encountered problems in delayed coking operations is foamover. Foamover typically results from the formation of an excessive volume of foam in the live coking drum during the fill cycle. When foamover occurs, partially coked resid is carried into the coke drum overhead line and, depending on the amount of such overflow, can result in: coke lay-down in the coke drum overhead lines; partial plugging of the combination tower bottoms screen; complete plugging of the combination tower bottom screen and a resultant sudden loss of feed to the coker heater; plugged (i.e., coked) heater tubes resulting from the sudden loss of flow therethrough; and plugging of the coker blowdown system. A massive foamover can even carry coke into the upper portions of the combination tower.
Foam is typically formed from condensed and/or entrained liquid hydrocarbons present in the live coking drum during the fill cycle. Condensate can form in the live drum when, for example, hot resid is first switched into the drum at the beginning of the fill cycle. Although the empty drum is typically warmed to a temperature in the range of from about 450.degree. to about 650.degree. F. prior to beginning the fill cycle, the warmed drum is still relatively cool compared to the hot resid material flowing from the coker heater. Thus, some condensation of vapor can occur, particularly on the interior surface of the coking drum.
Condensate can also accumulate in the empty coking drum during the warmup stage. During the warmup stage, a portion of the vapor product from the live coking drum is typically delivered downwardly through the empty drum. For a coking system operating on 20 hour cycles, the warmup stage will typically last for from about three to about four hours. At the beginning of the warmup stage, the temperature of the empty drum will typically be in the range of from about 200 to about 250.degree. F. Thus, a considerable amount of condensed hydrocarbon material can accumulate in the empty coking drum during the warmup operation.
One barrel of condensed and/or entrained hydrocarbon liquid material can form up to 1,200 barrels of foam in the live drum. The foam material travels up the coking drum on top of the coke layer. Several factors promote the formation and expansion of foam material within the filling drum. These include: the amount of condensed and/or entrained liquid hydrocarbon material present in the drum; pressure swings in the live coking drum, particularly significant pressure losses of the type which occur at the beginning of the fill cycle and when diverting vapor product to warm up an empty drum; a significant drop in overhead vapor product temperature; failure of the anti-foam chemical addition system; and over-filling the live drum.
The procedures heretofore used in the art for preventing foamovers have commonly included: attempting to ensure that the unit operators drain completely the warm, empty coking drum before beginning the fill cycle; injecting silicone anti-foam chemicals when a high foam level is detected in the live drum; restricting the fill rate so that the final level of the coke product is significantly below the top of the coking drum; and significantly limiting the amount of warmup vapor taken from the live drum.
The approaches used heretofore for reducing foam formation and expansion have serious shortcomings and are typically highly susceptible to operator error. Restricting fill rates and product levels significantly reduces unit capacity and, by necessitating the use of larger drums and/or a greater number of drums to achieve a given capacity, significantly increases construction costs. Silicone anti-foam chemicals are costly, unreliable, and can significantly poison catalysts used in fluid catalytic crackers and other downstream processing systems.
In addition, attempting to maintain live drum pressure by reducing the amount of warmup vapor taken from the live drum can result in the empty drum being not sufficiently warmed before beginning the fill cycle. Foam formation rates increase rapidly with decreasing switchover temperatures below 600.degree. F. Significantly restricting the rate of warmup vapor flow from the live drum can result in switchover temperatures of as low as 450.degree. F. Low switchover temperatures can produce large pressure and temperature losses at the beginning of the fill cycle.
Due to the fact that the coking drum product vapor constitutes the primary feed to the coking unit fractionator, the fractionator is also adversely affected by pressure, temperature, and product flow fluctuations in the coking drum. Such changes can easily upset the operation of the fractionator and thus have a deleterious effect on the consistency and quality of the product fractions drawn from the fractionator.